This invention relates generally to a vehicle control system, and more particularly to such a system in which if a sensor for detecting an operating condition of the vehicle is subjected to a malfunction, an actuator is controlled using backup data, instead of using the output of this sensor.
U.S. Pat. No. 4,450,812 (corresponding to Japanese Patent Publication No. 90/999/88) discloses a vehicle control system in which in the event of a malfunction of a sensor, backup data can be used. More specifically, this conventional control system comprises the sensor for detecting the condition of an engine, control means for controlling an actuator (here, a pulse motor for operating a fuel-air ratio control valve) in accordance with the output of the sensor so as to control the amount of supply of fuel to the engine, first judgment means for judging whether or not the output of the sensor is an abnormal value outside a predetermined normal range between upper and lower limit values, second judgment means for judging whether or not the abnormal output of the sensor determined by the first judgment means is lasting for a predetermined period of time, and backup data supply means. As shown in FIG. 5, when the first judgment means judges that the output V' of the sensor is an abnormal value above the upper limit value R.sub.U ' or below the lower limit value R.sub.L ' the backup data supply means supplies a value of the sensor output V', obtained immediately before this abnormal value, as backup data Y' to the control means. When the result of judgment of the second judgment means is "YES" (that is, the sensor output V' continues to be a normal for the predetermined period of time), the control means brings the engine into an idling condition, so that the vehicle can be guided to a safe side.
In the above conventional control system, when the sensor output V' goes outside the normal range due to noises, and then is returned to a normal value before the above predetermined period of time elapses, the sensor output V' is again used instead of the backup data Y' to thereby recover the normal control condition.
In the above conventional control system, however, as shown in FIG. 5, there are occasions when the backup data Y' is close to the upper limit value R.sub.U '. In this case, upon lapse of the above predetermined period of time, that is, when the control means is switched from the control based on the backup data Y' to the idling control, the operating condition of the vehicle is abruptly changed, which is undesirable. Also, when the control means controls the actuator in accordance with the backup data Y' close to the upper limit value R.sub.U ', the amount of supply of the fuel is increased to an almost maximum against the will of the driver, so that the engine speed is kept high for a predetermined time period, which is also undesirable.